[DevTools] Add protocol::Binary to ui_devtools/string_util.h.

This is in preparation of referencing this type from the code
generated by third_party/inspector_protocol/code_generator.py.

My overall WIP change is in here, and happy to explain more:
https://chromium-review.googlesource.com/c/chromium/src/+/1244719

Bug: chromium:891377
Change-Id: I2e59d49bbe333bb132ea8e16fcac2dace589f163
Reviewed-on: https://chromium-review.googlesource.com/c/1282224
Commit-Queue: Johannes Henkel <johannes@chromium.org>
Reviewed-by: Dmitry Gozman <dgozman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#600052}

components/ui_devtools/string_util.h

@@ -74,6 +74,19 @@ class StringUtil {
   static std::unique_ptr<Value> parseJSON(const String& string);
 };
 
+// A read-only sequence of uninterpreted bytes with reference-counted storage.
+// Though the templates for generating the protocol bindings reference
+// this type, thus far it's not used in the Chrome layer, so we provide no
+// implementation here and rely on the linker optimizing it away. If this
+// changes, look to content/browser/devtools/protocol_string{.h,.cc} for
+// inspiration.
+class Binary {
+ public:
+  const uint8_t* data() const;
+  size_t size() const;
+  String toBase64() const;
+  static Binary createFromBase64(const String& base64, bool* success);
+};
 }  // namespace protocol
 }  // namespace ui_devtools